Vikings RB Adrian Peterson (28) can lower his head near the line of scrimmage but not when he's running downfield. / Suchat Pederson, The (Delaware) News Journal

by Tom Pelissero, USA TODAY Sports

by Tom Pelissero, USA TODAY Sports

MANKATO, Minn. - Adrian Peterson came around on the new NFL rule barring players from leading with the crown of their helmets once he found out it didn't apply only to running backs.

But the league's reigning MVP can't be certain until games begin - and flags begin flying - how tightly officials will enforce the rule and what it will take for physical runners like him to adjust.

"It's going to be hard," Peterson told USA TODAY Sports after the Minnesota Vikings' training camp practice on Tuesday. "It's something a lot of guys have been doing - especially me - for a long time. You kind of adapt to that running style.

"But then again, you know that it's a rule now. And hopefully, it don't take you getting flags or pulling money out your pocket to make you really say, 'OK, now I've got to really make sure I'm doing that.' I'm sure some guys are going to take that. Hopefully, I'm not one of those guys."

Against the backdrop of lawsuits from thousands of former players who claim the NFL concealed the dangers of concussions, Commissioner Roger Goodell made the rule his primary health and safety initiative in March at the league meetings, where it passed by a 31-1 vote.

Green Bay Packers president Mark Murphy said then evidence was shown that dementia and other post-retirement health issues may occur more frequently among receivers, defensive backs and running backs "just because you've got the full-speed collisions."

The league hopes to cut down on those with the new rule, which prohibits any player from squaring up, lowering his body and delivering "a forcible blow" with the top (crown) of the helmet - though only plays outside the tackle box and at least 3 yards downfield qualify.

"In some aspects, you will change," Peterson said. "It's kind of crazy - they say within 3 to 4 yards you can have your head down. It doesn't really make sense. ... But the rules are the rules. You've just got to abide by it."

Violations, which won't be subject to replay review, will carry a 15-yard penalty. The violators could be subject to fines or even suspensions as well.

Only the Cincinnati Bengals voted against the rule in March, though officials for other teams expressed reservations, particularly about enforcement and how they're supposed to teach the altered technique. That included Vikings coach Leslie Frazier, whose star player was featured in the video shown before the final vote.

"We make (Peterson) aware of the rules, of course," Frazier told USA TODAY Sports this week. "But to say we're going to change his style of running - that's a hard deal, now. It's hard. We'll see what happens."'

Rich McKay, chair of the league's competition committee, said in March the committee studied every play from Weeks 10 through 16 last season and identified only 11 plays that would draw a flag under the new rule, with no repeat offenders.

Asked if Vikings coaches have made a point of emphasizing technique in light of the new rule, Peterson said, "Not really. They say something about it, of course. But they're not really focusing on it every day - 'make sure you keep your head up, make sure you keep your head up.' We know the consequences of it as far as getting a penalty."

Peterson said he understands the safety aspect as well, having seen or heard about enough players - particularly on defense - suffering neck injuries by putting their heads down, rather than lowering their pad level as they've been taught since childhood. He laughed off his starring role in the NFL's video, saying, "It was cool. I didn't mind at all."

The league has expressed confidence in officials' ability to get the call right by having three sets of eyes - one each front, back and to the side - on each play and encouraging them to conference before making a decision. Several of those officials will make their annual trip to Vikings camp for two practices next week, and it's a good bet Peterson might strike up a conversation.

The Vikings hire their own officials for every practice during camp. On Monday, during the first padded practice of camp, one of Peterson's teammates, undrafted rookie Bradley Randle, lowered his shoulders and blasted cornerback Greg McCoy to the ground. No flag was thrown.

"If that wasn't pretty much a crown, I don't know what is," Peterson said. "But I don't know, man. We'll just see how it plays out. I'm going to keep my head up as best I can - but I'm going to run the ball."